This invention relates to a diagnostic system for effectively testing the operation of sensors which sense the evaporator refrigerant pressure and the leaving chilled liquid temperature in a liquid chiller air conditioning system and for providing a warning when at least one of the sensors is found to be defective.
Large commercial and industrial air conditioning systems typically employ centrifugal liquid chillers. As the refrigerant flows through the system's evaporator, circulating liquid (usually water), which is in heat exchange relationship with the refrigerant, transfers heat to the refrigerant. The chilled liquid leaving the evaporator is then delivered to remote locations and used to cool a building or a zone. By maintaining the temperature of the leaving chilled liquid at a desired setpoint, the cooled space may be held at a desired temperature. The required control is usually accomplished by sensing the leaving chilled liquid temperature and adjusting the position of the guide vanes or prerotation vanes, at the inlet of the system's centrifugal compressor, in response to the sensed temperature. Adjusting the prerotation vanes varies the capacity of the centrifugal compressor, which in turn changes the refrigeration capacity of the system.
In addition to the sensor for sensing the leaving chilled liquid temperature, for safety reasons a sensor is usually provided to monitor the pressure of the refrigerant in the evaporator. If the evaporator pressure or the leaving chilled liquid temperature is too low, the chiller liquid passing over the evaporator tubes could freeze and cause damage to the air conditioning unit. Thus, by monitoring both the evaporator refrigerant pressure and the leaving liquid temperature, when either one of those variables drops below a minimum allowable level the unit may be shut down to prevent freezing of the circulating chilled liquid.
Of course, proper operation of the monitoring system requires valid information from the evaporator pressure sensor and from the leaving liquid temperature sensor. Unfortunately, in the past there was no way to check the individual sensors to verify or confirm that they were functioning properly. The failure of a sensor could go undetected and cause undesirable system operation or freeze-up without generating a system fault. If a sensor malfunctions there is no way of discovering this in the prior air conditioning systems.
This shortcoming has now been overcome by the present invention. By means of a relatively inexpensive arrangement, faulty evaporator pressure and leaving liquid temperature sensors are automatically detected and a fault warning message is displayed when a defective sensor is present.